Membrane gas separation is an energy efficient and environmentally friendly technology when compared to conventional cryogenic distillation or adsorption processes. Important gas pairs separated commercially by membrane processes include H2/N2 or H2/CH4 for H2 recovery, O2/N2 for O2 and N2 enrichment, CO2/CH4 for pre-combustion natural gas sweetening and CO2/N2 for post-combustion CO2 capture. To achieve both high flux and high gas selectivity, one key strategy is to fabricate ultra-thin gas selective layers, since the flux is inversely proportional to the thickness of a membrane. In this regard, good H2/CO2 separation performances have been reported for porous alumina supported metal organic framework (MOF) nanosheets and graphene oxides (GOs) atomic sheets. However, such membranes can only be prepared in small dimensions and remain extremely brittle, thus they can only operate at zero transmembrane pressure difference, which significantly restrains their practical applications. Porous SiNx frame-supported porous graphene films with superior mechanical sturdiness were fabricated by focused ion beam (FIB) perforation, but the relative large pores only afford very low gas selectivities. In contrast, polymer membrane-supported graphene and GOs films showed better gas selectivities, although low gas permeances (i.e., pressure normalized gas flux) for the coated polymer membranes were observed. To minimize the permeance loss while maximizing the gas selectivities, controlled oxidative surface modification methods (e.g., photo-oxidative and thermal oxidative) to optimize the gas separation performances of PIM-1 (i.e., a type of polymers of intrinsic microporosity, PIM, with nominal pore size less than 2 nm) have been proposed. The resulting membranes showed enhanced gas selectivities while maintaining high gas permeabilities. However, such methods are highly substrate-dependent.
There exists a need for films that demonstrate both high permeability and high gas selectivities at a wide range of transmembrane pressure differences (e.g., 1 to 10 bar), and scalable and substrate-independent techniques for making such films.